The present invention relates to the field of motor vehicle safety and deals, on the one hand, with an electropyrotechnic igniter and, on the other hand, with a pyrotechnic gas generator containing such an igniter, for inflating an airbag.
New generators known as xe2x80x9cadaptive generatorsxe2x80x9d have appeared recently with a view to limiting the risks of bodily injury caused by the deployment of the airbag. To do that, these various generators, coupled with appropriate detection systems, make it possible to alter the gas flow rate and/or the gas volume as a function of certain parameters, such as, for example, the morphology and the position of the passenger, the ambient temperature, the nature and the intensity of the impact. These generators are connected to an electronic control unit and generally have two combustion chambers, separated from one another by a sealed partition, each of which has its own pyrotechnic charge and its own ignition system. Each of the two ignition systems is then positioned at one of the opposite ends of the generator. Patent Application EP 0 382 552 describes a generator of this type.
In order to reduce the number of electrical connections between the electronic control unit and the generator, it has therefore been proposed, in Patent Application WO 97/16695 and in French Patent Application FR 2 763 548, for the two separate ignition systems to be replaced by an electropyrotechnic initiator with two ignition heads which is connected to the said electronic control unit via a single electrical supply. According to a first embodiment of this initiator, the latter contains two separate resistive heating elements incorporated in a switching bridge comprising four switches which can switch in pairs and which are combined with a routing circuit produced by a diode circuit. According to a second embodiment of this initiator, the latter comprises two different resistors mounted in parallel in an electric circuit. However, motor vehicle manufacturers wish, for safety reasons, to be able at any instant to check the value of each resistive element included in the initiator by successively passing electrical test currents generated using the electronic control unit. However, because, in the first embodiment of the initiator, the junction voltage of the diodes used is too high and depends on the temperature, it is very difficult to measure the value of the said resistive elements reliably. In the second embodiment of the initiator, in order to be able to test each of the two resistors, it is therefore necessary, because these resistors have different resistances, to vary the strength of the electrical current delivered by the electronic control unit. This is relatively complicated to do and, in addition, the use of two different resistors increases the number of steps needed in assembling the initiator and therefore the manufacturing costs.
An initiator has therefore been proposed, as described in French Patent Application FR 2 772 909, with two ignition heads comprising two resistive heating elements incorporated into an electric circuit which comprises three electrical connections, one being common to the two resistive elements and the other two each being connected to just one of the two resistive heating elements.
Nonetheless, given the great increase in the number of xe2x80x9cairbag modulesxe2x80x9d incorporated into a motor vehicle, an xe2x80x9cairbag modulexe2x80x9d consisting of a particular entity containing a gas generator associated with an airbag, the person skilled in the art, aside from being faced with a proliferation in the number of electrical connections, has noted that in the event of a collision, the electronic control unit was unable to deliver enough electrical energy to bring about the desired triggering of the various ignition systems included in the airbag modules.
In addition, depending on the nature and intensity of the impact, for example, it is desirable to be able to trigger only the appropriate airbag modules that will afford appropriate protection and therefore, in each airbag module finally adopted for protection, to be able to give totally different commands to each of the two ignition heads.
The person skilled in the art is still looking for an electropyrotechnic igniter with two ignition heads that makes it possible to overcome the aforementioned problems. Added to these are also the constraints associated with cost and bulk.
The object of the present invention is precisely to propose such an igniter and relates therefore to an electropyrotechnic igniter with two ignition heads, each of the said ignition heads comprising a body containing a resistive heating element and a pyrotechnic ignition composition, characterized in that
i) each of the two ignition heads comprises a means of dialogue and of triggering coded information and a means of storing electrical energy,
ii) the said igniter comprises external connection means intended to be connected electrically to an electronic control unit.
It therefore follows that the use of such an igniter in each of the various airbag modules incorporated into the vehicle and connected by a wired network of the bus type with an electronic control unit makes it possible, on the one hand, to leave the choice to the electronic control unit and to trigger only one or both of the two ignition heads included in the airbag module or modules likely to afford the occupant effective protection depending on the impact, and, on the other hand, to no longer ask the electronic control unit to deliver the amount of electrical energy needed to initiate such an igniter. This is achieved by virtue of the presence, in each of the two ignition heads of the igniter, on the one hand, of a means of storing electrical energy which is periodically supplied with low-strength electric current emitted by the electronic control unit and, on the other hand, of a dialogue and triggering means which is capable of detecting coded information from the electronic control unit and of giving the command to the said means of storing electrical energy to make it deliver the amount of electrical energy hitherto stored. This energy makes it possible to heat the resistive heating element through the Joule effect, so as to initiate the pyrotechnic ignition composition.
As a preference, the dialogue and triggering means will be a specific integrated circuit and the coded information which flows between the electronic control unit and the said dialogue and triggering means will consist of strings of specific electrical pulses. In the current application, the coded information will comprise both information constituting the command to trigger the two ignition heads of the igniter and information allowing the electronic control unit to satisfy itself of the reliability of the electronic components contained in each of the two ignition heads of the igniter.
Advantageously, in each of the two ignition heads, a transverse dividing wall splits the interior of the body into, on the one hand, a downstream chamber containing the resistive heating element and the pyrotechnic ignition composition, and, on the other hand, an upstream chamber containing an electronic board onto which the dialogue and triggering means and the means of storing electrical energy are integrated.
Advantageously also, in each of the two ignition heads, electrical connection means pass through the transverse dividing wall and connect the resistive heating element to conducting tracks which are formed on the electronic board. As a preference, the transverse dividing wall is made using a metal part having several perforations, each of these perforations having a glass side wall. Advantageously, the metal part has two perforations and the electrical connection means consist of two metal connecting pins, each inserted into one of the two perforations borne by the metal part and having a first end attached to the resistive heating elements and a second end soldered to the conducting tracks. This transverse dividing wall therefore provides sealing between the upstream chamber and the downstream chamber before and after the ignition head is operated and also electrically isolates the metal connecting pins from one another.
As a preference, the two electronic boards are arranged at right angles to their respective transverse dividing wall. To improve the mechanical strength of the said electronic boards and of the electronic components fixed on them, it is desirable for the assembly to be covered with overmoulding or a coating.
Advantageously, in each of the two ignition heads, the dialogue and triggering means is placed on one of the two flat faces of the electronic board, and the means of storing electrical energy is placed on the other flat face.
Advantageously, the external connection means consist of external metal pins, each of which has, on the one hand, a first part which is arranged in the continuation of the two electronic boards and which is parallel to these electronic boards, and, on the other hand, a second part which is at right angles to the first part.
As a preference, the resistive heating elements each consist of a thin-film resistive bridge, the latter consisting of tantalum nitride, the thickness of which is between 0.01 xcexcm and 1 xcexcm. The resistive bridge could just as easily consist, for example, of a layer of a nickel/chromium alloy, the thickness of which is between 0.01 xcexcm and 1 xcexcm.
The invention also relates to the use of such an electropyrotechnic igniter in a gas generator which is intended to inflate an airbag and which has an elongate tubular body. This igniter is then preferably fixed into the central part of the generator body, that is to say between the two combustion chambers thereof, and itself provide thermal and mechanical isolation between these two combustion chambers.